Advances in Solid-State Lasers: Development and Applications 192 6. References Jin, J.; Kim, Y. -J.; Kim, Y.; Kim, S. –W. & Kang, C. –S. (2006) Absolute length calibration of gauge blocks using the optical comb of a femtosecond pulse laser, Optics Express, Vol. 14, No. 13, pp. 5568-5974, ISSN 1094-4087 Jin, J.; Kim, Y. -J.; Kim, Y. & Kim, S. –W. (2007) Absolute distance measurement using the optical comb of a femtosecond pulse laser, International Journal of Precision Engineering and Manufacturing, Vol. 8, No. 7, pp. 22-26, ISSN 1229-8557 Kim, Y. -J.; Jin, J.; Kim, Y. ; Hyun, S. & Kim, S. –W. (2008) A wide-range optical frequency generator based on the frequency comb of a femtosecond laser, Optics Express, Vol. 16, No. 1, pp. 258-264, ISSN 1094-4087 Hyun, S.; Kim, Y. -J.; Kim, Y. ; Jin, J. & Kim, S. –W. (2009) Absolute length measurement with the frequency comb of a femtosecond laser, Measurement Science and Technology, Vol. 20, pp. 095302-1-6, ISSN 0957-0233 Minoshima, K. & Matsumoto, H. (2000) High-accuracy measurement of 240-m distance in an optical tunnel by use of a compact femtosecond laser, Applied Optics, Vol. 39, No. 30, pp. 5512-5517, ISSN 0003-6935 Yamaoka, Y.; Minoshima, K. & Matsumoto, H. (2002) Direct measurement of the group refractive index of air with interferometry between adjacent femtosecond pulses, Applied Optics, Vol. 41, No. 21, pp. 4318-4324, ISSN 0003-6935 Bitou, Y.; Schibli, T. R.; Minoshima, K. (2006) Accurate wide-range displacement measurement using tunable diode laser and optical frequency comb generator, Optics Express, Vol. 14, No. 2, pp. 644-654, ISSN 1094-4087 Schibli, T. R.; Minoshima, K.; Bitou, Y.; Hong, F. –L.; Bitou, Y.; Onae, A. & Matsumoto, H. (2006) Displacement metrology with sub-pm resolution in air based on a fs-comb wavelength synthesizer, Optics Express, Vol. 14, No. 13, pp.5984-5993, ISSN 1094- 4087 Bitou, Y. & Seta K. (2000) Gauge block measurement using a wavelength scanning interferometer, Japanese Journal of Applied Physics, Vol. 39, pp. 6084-6088, ISSN 0021- 4922 Schibli, T. R.; Minoshima, K.; Hong, F. –L.; Inaba, H.; Onae, A.; Matsumoto, H.; Hartl, I.; Fermann, M. E. (2004) Frequency metrology with a turnkey all-fiber system, Optics Letters, Vol. 29, No. 21, pp.2267-2469, ISSN 0146-9592 Lay, O. P.; Dubovitsky, S.; Peters, R. D. & Burger, J. P (2003) MSTAR: a submicrometer absolute metrology system, Optics Letter, Vol. 28, pp. 890-892, ISSN 0146-9592 Walsh, C. J. (1987) Measurement of absolute distances to 25 m by multiwavelength CO 2 laser interferometry, Applied Optics, Vol. 26, No. 9, pp. 1680-1687, ISSN 0003-6935 Bien, F.; Camac, M,; Caulfield, H. J. & Ezekiel, S. (1981) Absolute distance measurements by variable wavelength interferometry, Applied Optics, Vol. 20, No. 3, pp. 400-403, ISSN 0003-6935 Dändliker, R.; Thalmann, R. & Rregué (1988) Two-wavelength laser interferometry using super heterodyne detection, Optics Letters, Vol. 13, No. 5, pp. 339-341, ISSN 0146- 9592 Precision Dimensional Metrology based on a Femtosecond Pulse Laser 193 Kubota, T.; Nara, M. & Yoshino, T. (1987) Interferometer for measuring displacement and distance, Optics Letters, Vol. 12, No. 5, pp. 310-312, ISSN 0146-9592 Jost, J. D.; Hall, J. L. & Ye, J. (2002) Continuously tunable, precise, single frequency optical signal generator, Optics Express, Vol. 10, pp. 515-520, ISSN 1094-4087 Birch, K. P. & Downs, M. J. (1993) An updated Edlen equation for the refractive index of air, Metrologia, Vol. 34, pp. 479-493, ISSN 0026-1394 Schuhler, N.; Salvadé, Y.; Lévêque, S.; Dändliker, R & Holzwarth, R. (2006) Frequency- comb-referenced two-wavelength source for absolute distance measurement, Optics Letters, Vol. 31, No. 21, pp. 3101-3103, ISSN 0146-9592 Slavadé, Y.; Schuhler, N.; Lévêque, S. & Floch, S. L. (2008) High-accuracy absolute distance measurement using frequency comb referenced multiwavelength source, Applied Optics, Vol. 47, No. 14, pp. 2715-2720, ISSN 0003-6935 Coddington, I; Swann, W. C.; Nenadovic, L. & Newbury, N. R. (2009) Rapid and precise absolute distance measurement at long range, Nature Photonics, Vol. 3, pp. 351-356, ISSN 1749-4885 Jin, J.; Kim, Y. -J.; Kim, Y. ; Hyun, S. & Kim, S. –W. (2008) Absolute distance measurement using the frequency comb of a femtosecond pulse laser, Proceedings of the European Society of Precision Engineering and Nanotechnology (EUSPEN) International conference, O7.2, Zurich, 05/2008, EUSPEN, Cranfield Jin, J.; Kim, Y. -J.; Kim, Y. ; Hyun, S. & Kim, S. –W. (2007) Precision length metrology using and optical frequency generator, Proceedings of the Asian Society of Precision Engineering and Nanotechnology (ASPEN) 2007, pp. 39-41, Kwangju, 11/2007, KSPE, Seoul (invited) Jin, J.; Kim, Y. -J.; Kim, Y. ; Hyun, S. & Kim, S. –W. (2007) Precision length metrology using and optical frequency synthesizer, Proceedings of the Conference on Laser and Electro-Optics-Pacific Rim (CLEO-PR), pp. 1443-1444, Seoul, 08/2007, OSA, Seoul (invited) Jin, J.; Kim, Y. -J.; Kim, Y. & Kim, S. –W. (2006) Absolute length metrology using a femtosecond pulse laser, Proceedings of the 2 nd International Conference on Positioning Technology, pp. 119-121, Daejeon, 10/2006, KSPE, Seoul Jin, J.; Kim, Y. -J.; Kim, Y.; Kim, S. –W. & Kang, C. -S (2006) Absolute length calibration of gauge blocks using optical comb of a femtosecond pulse laser, Proceedings of SPIE Optics & Photonics, pp. 6292O-1, San-diego, 08/2006, SPIE, Bellingham Jin, J.; Kim, Y. -J.; Kim, Y.; Kim, S. –W. & Kang, C. -S (2006) Absolute length calibration of gauge blocks using optical comb of a femtosecond pulse laser, Proceedings of the European Society of Precision Engineering and Nanotechnology (EUSPEN) International conference, pp. 410-414, Baden, 05/2006, EUSPEN, Cranfield Kim, S. –W.; Oh, J. S.; Jin, J.; Joo, K. N. & Kim, Y. -J. (2005) New precision dimensional metrology using femtosecond pulse lasers, Proceedings of the European Society of Precision Engineering and Nanotechnology (EUSPEN) International conference, pp. 135- 138, Montpellier, 05/2005, EUSPEN, Cranfield Kim, S. –W.; Joo, K. N.; Jin, J. & Kim, Y. -J. (2005) Absolute distance measurement using femtosecond laser, Proceedings of SPIE, pp. 58580N-1-8, Munich, 06/2005, SPIE, Bellingham Advances in Solid-State Lasers: Development and Applications 194 Ye, J. & Cundiff, S. T. (2005). Femtosecond optical frequency comb: Principle, operation, and applications, Springer, ISBN 0-387-23790-9, New York Rulliére, C. (1998). Femtosecond laser pulses; Principles and experiments, Springer, ISBN 3-540- 63663-3, Berlin Heidelberg 10 Micro-Solid-State Laser for Ignition of Automobile Engines Masaki Tsunekane, Takayuki Inohara, Kenji Kanehara and Takunori Taira Institute for Molecular Science, Nippon Soken, Inc. Japan 1. Introduction Recently in consideration of the problem of protecting the global environment and preserving fossil resources, the research and development of new clean vehicles driven by clean energy sources, such as electricity, fuel cell, etc., has been progressing worldwide. However it is difficult to replace all conventional gasoline vehicles to clean vehicles immediately, because they still have several hurdles to get over, costs of the clean vehicles and the energy sources, range between refuelling, the availability of refuelling or recharging stations, vehicle performance, fuel cell lifetime, etc. Therefore the improvement of the efficiency of conventional internal combustion gasoline engines, and the reductions of CO 2 and harmful pollutant emissions have become more important today. A laser has been discussed widely as one of the promising alternatives for an ignition source of the next generation of efficient internal combustion engines (Hickling & Smith, 1974; Dale et al., 1997; Phuoc, 2006). Laser ignition can change the concept of ignition innovatively and has many advantages over conventional electric spark plug ignition. Figure 1 shows the schematics of combustion engines ignited by (a) an electric spark plug and a laser (b), (c). (a) (b) (c) Fig. 1. Schematics of the combustion engines ignited by (a) a spark plug and (b), (c) a laser. (c) shows multipoint ignition. Using a laser, the ignition plasma may be located anywhere within the combustion chamber because laser ignition doesn’t need electrodes. Optimal positioning of ignition apart from Advances in Solid-State Lasers: Development and Applications 196 the cold cylinder wall allows the combustion flame front to expand rapidly and uniformly in the chamber and thus increases the efficiency as seen in (b). In addition, laser ignition has great potential for simultaneous, spatial multipoint ignition within a chamber as shown in (c). This shortens combustion time dramatically and improves the output and efficiency of engines effectively (Phuoc, 2000; Morsy et al., 2001). Further a laser can ignite leaner or high- pressure mixtures that are difficult to be ignited by a conventional electric spark plug (Weinrotter et al., 2005). A laser igniter is also expected to have a longer lifetime than a spark plug due to the absence of electrodes. One of the major difficulties of laser ignition for actual applications, especially for automobiles is the dimension of the lasers. For breakdown in fuel mixtures, light intensities in the order of 100GW/cm 2 are necessary at the focal point of ignition. Then lasers with pulse energies higher than 10mJ, beam quality factors, M 2 of lower than 3 and pulse durations of shorter than 10ns have been used for combustion experiments. But the commercially available laser heads have table top size due to the complexities of the laser cavity and the cooling system. In reductions of system size and costs, a multiplexing fiber optics delivery system seems to be ideal and practical for laser ignition of multicylinder engines. But it is difficult to deliver ignition light through fibers to each cylinder of an engine directly, because the optical damage threshold of fibers is still several orders of magnitude less than the peak energy levels required by laser ignition at present (Joshi et al., 2007). The fundamental problem of a fiber is attributed to the need to deliver relatively high power pulses with sufficient beam quality to focus the output light to the intensity required for breakdown. 2. Characteristics of passively Q-switched laser A passively Q-switched solid-state laser, especially a Nd:YAG/Cr 4+ :YAG laser end-pumped by a fiber-coupled laser diode (LD) has been proposed as a promising ignition laser recently (Kofler et al., 2007). It has a simple structure, only two functional optical elements and no external power for optical switching is necessary hence the dimension of the laser head can be reduced. In addition, a short pulse operation less than 1ns is easily obtained by reduction of the cavity length less than 10mm and the beam quality is also good due to the soft aperture effect of a Cr:YAG saturable absorber (Zaykowski & Dill III, 1994; Sakai, 2008). The fiber delivered pump system allows not only further size reduction but also reliable laser operation, because the pump LD which is very sensitive to environmental temperature can be positioned at relatively stable place inside a car apart from the hot engine. In generally, passively Q-switched lasers have large pulse-to-pulse energy fluctuations and large timing jitters under continuous-wave (CW) pumped operations (Huang et al., 1999; Tang et al., 2003) due to thermal and mechanical instabilities. Such fluctuations and jitters have strongly restricted the applications of passively Q-switched lasers. On the other hand, operation frequency of igniters of internal combustion engines is less than 60Hz, corresponding to an engine speed of 7200 rpm, and the duty cycle is less than 5% for automobiles. In such a low frequency, quasi-continuous-wave (QCW) pumped operations with a low duty cycle, passively Q-switched lasers are expected to operate stably due to initialization of the thermal and mechanical conditions during pulses. The characteristics of passively Q-switched lasers have been analyzed in detail and various optimum design criteria have been presented (Szabo & Stein, 1965; Degnan, 1995; Xiao & Bass, 1997; Zhang et al., 1997; Chen et al., 2001; Pavel, 2001; Patel & Beach, 2001). But there Micro-Solid-State Laser for Ignition of Automobile Engines 197 are still several discrepancies in the theoretical calculations and the experimental results especially for output energy and efficiency. We think that the main cause is uncertainness of size of the laser mode. It is not easy to estimate the actual laser mode size and the beam quality accurately, because the aperture formed in the saturable absorber of a Cr:YAG crystal has a complex spatial distribution of transmission and it changes dynamically (Zabkar et al., 2008). In this paper we demonstrated the optimum design of a high-brightness (high peak power and high beam quality), passively Q-switched micro-solid-state laser for ignition of engines. The performance of the micro-laser including fluctuations of pulse-to-pulse energy and timing jitters in QCW pumped operations was evaluated in detail. The combustion experiments in a static test chamber and a dynamic real automobile engine ignited by the micro-laser were demonstrated and discussed compared to a conventional spark plug. From the results, we could confirm that a high-brightness laser could dramatically reduce the minimum ignition energy and we also found that multi-pulse (pulse-train) ignition was effective to improve ignition possibility for leaner mixtures. Finally we successfully demonstrated the prototype laser igniter which had the same dimension as a spark plug including all optics for ignition. 3. Performance of micro-solid-state laser for ignition 3.1 Performance of passively Q-switched micro-laser module Figure 2 shows (a) a schematic drawing and (b) a photograph of a passively Q-switched micro-laser module (Tsunekane et al., 2008). An active medium of a 1.1at.% Nd doped YAG crystal (crystal orientation of <111>, Sumitomo Metal Mining Co., Ltd.) with a length of 4mm is longitudinally pumped by a fiber-coupled, conductive-cooled, 120W (peak) QCW 808nm laser diode (JENOPTIK laserdiode GmbH). The core diameter of the fiber is 0.6mm with N.A. of 0.22. The pump light from the fiber was collimated by a lens set to have a diameter of 1.1mm in the active medium. Antireflection (<0.2%) and high-reflection (>99.8%) coatings at 808nm and 1064nm, respectively, were deposited on the pumped surface of the Nd:YAG crystal. High-reflection (>90%) and antireflection (<0.2%) coatings at 808nm and 1064nm, respectively, were deposited on the other, intra-cavity surface of the (a) (b) Fig. 2. (a) Schematic drawing and (b) photograph of the passively Q-switched micro-laser module. Advances in Solid-State Lasers: Development and Applications 198 crystal. The high-reflection coating at 808nm makes efficient pump absorption possible by a round trip path of the pump light and it can also prevent a closely situated Cr:YAG crystal from pump-induced breaching (Zaykowski & Wilson, Jr., 2003; Jaspan et al., 2004). Antireflection (<0.2%) coatings at 1064nm were deposited on both surfaces of the saturable absorber of a Cr 4+ :YAG crystal with a length of ~4mm (crystal orientation of <100>, Scientific Materials Corp.). The output coupler is flat with a reflectivity of 50% at 1064nm. The cavity length is 10mm. These optical elements were aligned carefully with an output coupler and fixed in the conductive-cooled, temperature-stabilized module (40mm-width × 28mm-height × 61mm-length) as shown in the figure. The module does not include focusing optics of the output beam for breakdown. In the following experiments, the pump energy was controlled by changing the pump duration with the peak pump power maintained at 120W. The maximum pump duration is 500µs limited by the diode. The repetition rate was 10Hz constant. Figure 3 (a) shows the output energy of the passively Q-switched micro-laser as a function of the initial transmission of a Cr:YAG crystal. The output of a passively Q-switched laser forms a pulse train, which is well known. The closed circles and the solid line denote the experimental values and the calculation of pulse energy (energy per pulse), respectively, and the open circles denote the experimental values of the total output energy (sum of pulse energies) at a pump duration of 500µs. The pulse energy increases to 4.3mJ as an initial transmission of the Cr:YAG crystal decreases to 15%. On the other hand, the total output energy decreases from 25 to 12mJ as an initial transmission decreases from 80% to 15%, because the pulse-to-pulse interval becomes longer and then the number of laser pulses decreases even though the pulse energy increases. The decrease in total output energy simply means the decrease in efficiency of the laser. Figure 3 (b) shows the pulse width as a function of the initial transmission of a Cr:YAG crystal. The pulse width was measured by a 10GHz InGaAs detector (ET-3500, Electro- Optics Technology, Inc,) with a 12GHz oscilloscope (DSO81204B, Agilent Technology). The closed circles and broken line denote the experimental values and the calculation of the pulse width, respectively. The pulse width decreases as the initial transmission decreases. The shortest pulse width of 300ps was obtained at an initial transmission of 15%. a) (b) Fig. 3. (a) Output energy and (b) pulse width of the passively Q-switched micro-laser as a function of the initial transmission of Cr:YAG. Micro-Solid-State Laser for Ignition of Automobile Engines 199 In the theoretical calculations shown in Fig.3, we assumed the ground-state absorption cross section of Cr 4+ :YAG as σ SA =2×10 -18 cm 2 and the excited-state absorption as σ ESA =5×10 -19 cm 2 . These are very important parameters but vary greatly in previous reports hence we used the averaged values in recent reports (Burshtein et al., 1998; Xiao et al., 1999; Feldman et al., 2003). In the calculation of pulse energy, we also assumed that the laser mode has the same size as the pump beam. Theoretically the output pulse energy is proportional to the area of the laser mode, so it is understood that the actual laser mode size is smaller by 10% or more than the pump beam due to the aperture effect of the saturable absorber of a Cr:YAG crystal as shown in Fig.3(a). As the initial transmission is higher, the discrepancy is larger. On the other hand, the calculation of pulse width agrees well with the experimental results as seen in Fig.3 (b), because the calculation of pulse width has no relation to the size of the modes. Though the highest pulse energy and shortest pulse width were obtained at an initial Cr:YAG transmission of 15%, optical damage was observed at the output coupler and then the beam quality was degraded. The beam quality was also degraded at initial transmissions of higher than 70%, because the aperture effect of a Cr:YAG crystal is weak. To use a laser for ignition, optical intensities of the order of 100GW/cm 2 are necessary at the focal point for breakdown. From our experimental observations, stable breakdown in air was observed at a pulse energy larger than 1.5mJ and a pulse width less than 1ns using an aspheric focus lens with a focal length of 10mm. In addition to the pulse energy, the total output energy is also necessary to ignite fuel-lean mixtures as discussed later. However as seen in Fig.3(a) the pulse energy and the total output energy are in the conflicting relation. Therefore we selected 30% as an optimum initial transmission of a Cr:YAG saturable absorber in our laser configurations. The laser performances were tested in detail and finally it was applied to the combustion experiments in the optimum condition. Figure 4 shows the laser output energy and optical-to-optical conversion efficiency as a function of pump duration at an initial Cr:YAG transmission of 30%. As a unique characteristic of passively Q-switched lasers, the output pulse energy is constant until the following pulse is generated, then the output characteristic changes to the shape of stairs. The interval of pulse generation is almost constant at 100μs. The output energy obtained was 2.7mJ per pulse and totally 11.7mJ (sum of 4 pulses) was obtained at a pump duration of 500μs. The optical-to- optical conversion efficiency changes largely and periodically by the pump duration and the maximum efficiency of 19% was obtained at the durations of pulse generation. Fig. 4. Output energy and optical-to-optical conversion efficiency as a function of pump duration at an initial transmission of 30%. Advances in Solid-State Lasers: Development and Applications 200 Figure 5 shows the fluctuation of the total output energy as a function of pump duration which was estimated statistically from 500 consecutive pulses. The perpendicular doted lines show the pump durations at which the number of pulses increases as shown in Fig.4. It is understood that the fluctuation increases by increase in the number of pulses and at the duration when the number of pulses changes. The fluctuation is still less than 100µJ (3%). Fig. 5. Fluctuation of the total output energy as a function of pump duration which was estimated statistically from 500 consecutive pulses. Figure 6 shows (a) the delay times of the each laser pulse from the standup of the pump LD current and (b) the jitters (standard deviation) of the delay times estimated statistically from 500 consecutive pulses as a function of pump duration. As seen in Fig.6 (a), the delay times are not dependent on the pump duration. The pulse interval is constant around 100µs and is equal to the interval of pulse generation. On the other hand, the jitter of the delay time strongly depends on the pump duration and also on the number of pulses. The first pulse has a small jitter for 200ns or less, and it is not dependent on pump duration, but the pulses generated later have a large jitter of 1µs or more, and the jitter changes sharply with the a) (b) Fig. 6. (a) Delay times of the each laser pulse from the standup of the pump LD current and (b) jitters (standard deviation) of the delay times estimated statistically from 500 consecutive pulses, as a function of pump duration [...]... width of the input seed The beam width may be comparable with the distance between the centre of the beam and the edge of the slab input face, i.e θL/2 On the other hand, when increasing the angle θ there is a possibility that the beam width becomes comparable with its distance from the other edge of the slab, of width D Therefore, the effective beam diameter to be amplified is the minimum out of the quantities... conditions, because of the strong gain shaping of the pump beam in the high-gain amplifier and of the great mismatch between the seed diameter and the gain-sheet thickness, the beam divergence along the y-direction is nearly pump-insensitive Considering a flat-top y-profile for the output beam leaving the amplifier, the thickness W of the gain-sheet can be readily estimated by measuring the full vertical... double pass and also employing a single 300-mm focal spherical lens to focus the seed into the Nd:YVO4 slab instead of the cylindrical optics shown in Fig 6 Both the amplifier gain as a function of the injected seed power and the output power as a function of the absorbed pump power are summarised in Figs 8 and 9 Fig 8 Gain curves for the single- and double-pass amplifier (A) Setup with pump focusing with... choose the seed waist 224 Advances in Solid-State Lasers: Development and Applications inside the amplifier, both the thickness of the pumped region and the Rayleigh range of the focused beam should be taken into account In particular the pumped region length along the seed propagation direction should be shorter than two times the Rayleigh range of the focused beam in order to take advantage of the entire... by thin indium foils A proper choice of the water temperature set point is crucial in optimising the amplifier performances The reason is twofold: i) minimisation of the thermal stress High Gain Solid-State Amplifiers for Picosecond Pulses 223 inside the slab and ii) reduction of the thermal red-shift of the fluorescence bandwidth of Nd:YVO4, that may reduce the amplifier gain and performance In this... sθ The propagation inside the amplifier occurs with a nearly-constant beam cross section in order to optimise the overlap efficiency, therefore the gain can be calculated along the longitudinal s coordinate as in a ray-tracing approximation: dI (ξ , y) ds = σ n(x, y)I (ξ , y) (1) 2 16 Advances in Solid-State Lasers: Development and Applications Fig 1 Model of the slab amplifier (seen from above): the. .. continuously and stably to a large flame On the other hand, the flame generated by a laser moves randomly in the free space of the chamber by turbulent flow during the growth The contamination and damage of an optical window in the combustion chamber are wellknown serious problems of laser ignition (Ranner et al., 2007) In our experiments, Al2O3 was used as a window material No visual contaminations and. .. 2λL/W In this example it turns out W ≈ 70 μm Notwithstanding the clearly non optimised beam size, the seed beam yields up to 3.5 W in a single pass through the amplifier for an optimum internal grazingincidence angle of ≈ 3° The setup for the cw amplification experiments is shown in Fig 6 Fig 6 Setup for the cw amplifier CL1: 250-mm focal cylindrical focusing lens; CL2: 100-mm focal cylindrical focusing... cylindrical pump focusing lens; HWP: half-wave plate; CML: collimation microlens (0.9-mm focal length) Being the thickness W of the gain sheet set by the pump focusing lens and limited by the residual smile of the laser diode, the depth of the gain sheet in the horizontal plane is basically determined by the pump absorption depth, 1/αP ≈ 0.5 mm, given the typical pump spectrum width (≈ 2-3 nm) and the. .. if the following pulses were not injected In the case of spark plug ignition shown in the right end, the ignition probability is below 100% even in an A/F of 15.7, which is slightly leaner than in a stoichiometric mixture Schlieren photography also demonstrated that the growth of the flamee kernel was slow and the time during the boundary of the flame kernel in contact with the electrodes was long, and . from Advances in Solid-State Lasers: Development and Applications 1 96 the cold cylinder wall allows the combustion flame front to expand rapidly and uniformly in the chamber and thus increases. seen in Fig .6 (a), the delay times are not dependent on the pump duration. The pulse interval is constant around 100µs and is equal to the interval of pulse generation. On the other hand, the. 61 mm-length) as shown in the figure. The module does not include focusing optics of the output beam for breakdown. In the following experiments, the pump energy was controlled by changing the